1. Field of the Invention
The present invention relates to a four-wheel drive vehicle including a pair of right and left main driving wheels and a pair of right and left auxiliary driving wheels. The present invention also relates to a control method and a control apparatus for the four-wheel drive vehicle.
2. Description of the Related Art
Hitherto, there is a four-wheel drive vehicle including a pair of right and left main driving wheels to which a driving force of a drive source is constantly transmitted, and a pair of right and left auxiliary driving wheels to which the driving force of the drive source is transmitted via a propeller shaft in a disconnectable manner. When this type of four-wheel drive vehicle travels in a two-wheel drive mode in which the driving force of the drive source is transmitted only to the main driving wheels, the four-wheel drive vehicle may stop rotation of the propeller shaft by interrupting a driving force transmission path on a vehicle front side and a vehicle rear side of the propeller shaft (see, for example, Japanese Patent Application Publication No. 2016-74370 (JP 2016-74370 A)). In this four-wheel drive vehicle, power loss caused by rotational resistance due to the rotation of the propeller shaft can be suppressed in the two-wheel drive mode, whereby the fuel efficiency can be improved.
In the four-wheel drive vehicle described in JP 2016-74370 A, a first connection/disconnection mechanism is provided in a transfer arranged on the vehicle front side of the propeller shaft, and a second connection/disconnection mechanism is provided between the rear wheels and a rear differential arranged on the vehicle rear side of the propeller shaft. Each of the first connection/disconnection mechanism and the second connection/disconnection mechanism includes a dog clutch and a synchronization mechanism for meshing the dog clutch. When the four-wheel drive vehicle travels in the two-wheel drive mode, the driving force transmission path from the drive source (engine) is interrupted by the first connection/disconnection mechanism and the second connection/disconnection mechanism, and the rotation of the propeller shaft is stopped.
When the two-wheel drive mode is switched to a four-wheel drive mode, the propeller shaft is rotated by a frictional force generated by a synchronization ring of the synchronization mechanism of each of the first connection/disconnection mechanism and the second connection/disconnection mechanism. When rotational members coupled by the dog clutch are synchronized, the dog clutch is meshed to achieve the four-wheel drive mode in which the driving force is transmitted to the rear wheels serving as the auxiliary driving wheels via the propeller shaft. In the four-wheel drive mode, the driving force to be distributed toward the rear wheels is adjusted by an electronic control coupling arranged between the propeller shaft and the second connection/disconnection mechanism.
In the four-wheel drive vehicle described in JP 2016-74370 A, the rotational synchronization is achieved in the dog clutch by rotating the propeller shaft by the frictional force generated by the synchronization ring. Therefore, the two-wheel drive mode cannot quickly be switched to the four-wheel drive mode unless the torque transmission capacity of the synchronization mechanism is increased. In order to increase the torque transmission capacity of the synchronization mechanism, for example, the size of the synchronization ring needs to be increased, which may cause an increase in the size and weight of the apparatus.